Composite sensor membrane

ABSTRACT

A sensor may include a membrane to deflect in response to a change in surface stress, where a layer on the membrane is to couple one or more probe molecules with the membrane. The membrane may deflect when a target molecule reacts with one or more probe molecules.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/426,851, filed Nov. 15, 2002, entitled “MULTIPLEXED BIOMOLECULARANALYSIS,” which is hereby incorporated by reference.

STATEMENT OF GOVERNMENT-SPONSORED RESEARCH

This invention was made with Government support under Grant (Contract)No. R21 CA86132-01 awarded by the National Institutes of Health/NationalCancer Institute and Contract No. DE-FG03-98ER14870 awarded by theUnited States Department of Energy. The Government has certain rights inthis invention.

TECHNICAL FIELD

This disclosure relates to sensors such as physical, chemical, andbiological sensors.

BACKGROUND

Micro-electromechanical (MEMS) sensors may use microcantilevers to sensephysical, chemical, and biological interactions. A microcantilever is astructure that is fixed at one end and free at the other. MEMSfabricated microcantilevers may be fabricated using silicon-basedmaterials.

For example, microcantilever sensors may be used to sense biomolecularinteractions as follows. In order to identify particular biologicalmolecules (referred to as target molecules), a surface of amicrocantilever may be functionalized with a particular probe molecule,where the probe molecule interacts with the target molecule. Forexample, in order to detect particular DNA material, a shortsingle-stranded DNA (ssDNA) sequence may be used as a probe molecule fora complimentary ssDNA. Similarly, in order to detect a particularantigen, an appropriate antibody may be used as a probe molecule.

FIGS. 1A and 1B illustrate biological sensing using a microcantilever.Referring to FIG. 1A, a cantilever 100 is fixed at a first end 110 to asubstrate 190, and free to move at a second end 120. A region 130 of thecantilever includes one or more probe molecules 140 for sensing targetmolecules. FIG. 1A shows the cantilever in its undeflected state.

Referring to FIG. 1B, a target molecule 150 may interact with one ormore of probe molecules 140, changing the surface stress of cantilever100 and causing cantilever 100 to bend. The amount by which cantilever100 bends generally depends on the number of target molecules 150interacting with probe molecules 140, and may therefore provide ameasure of the concentration of target molecules 150. The deflection ofthe cantilever may be detected using, for example, optical orpiezoresistive detection techniques.

SUMMARY

In general, in one aspect, a sensor may include a membrane to deflect inresponse to a change in surface stress. A layer on the membrane may beprovided to couple one or more probe molecules with the membrane. Themembrane may deflect when a target molecule reacts with one or moreprobe molecules. The membrane may be fixed to a substrate at a firstportion and a second different portion, and may span a well in thesubstrate.

The membrane may include a flexible material, such as a polymer.Polymers such as polyimide and parylene, or other polymers may be used.The layer may include a material to couple probe molecules to themembrane. For example, the layer may include gold. The layer may cover aportion of a first side of the membrane. The portion may be betweenabout 5% and about 90%, or between about 10% and about 70%.

A system may include a substrate and one or more membranes coupled withthe substrate. For example, the system may include a membrane spanning awell, where the membrane may have a layer to couple probe molecules tothe membrane. The system may also include another membrane spanninganother well, where the another membrane has a layer to couple probemolecules with the membrane. The system may include a cover to enclosethe well and the another well. The system may include channels toprovide fluid to the membranes.

In general, in another aspect, a method may include introducing fluidinto a region proximate to a membrane, the fluid including one or moretarget molecules to be sensed. At least some of the target molecules mayinteract with the probe molecules and cause the membrane to deflect. Themethod may include measuring the deflection of the membrane. Thedeflection may be measured using optical detection methods and/orelectrical detection methods.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show a cantilever in the undeflected and deflectedpositions.

FIGS. 2A and 2B show different implementations of membranes that may beused.

FIG. 3 illustrates measuring membrane deflection using optical beamdeflection.

FIG. 4A shows a Fabry-Perot interferometry apparatus.

FIG. 4B shows a Michelson interferometry apparatus.

FIG. 5 shows a membrane spanning a well in a substrate.

FIGS. 6A to 6E illustrate fabrication of a membrane structure such asthat shown in FIG. 5.

FIGS. 7A and 7B show top and side views of a system including multiplesensors.

FIGS. 8A and 8B show a cross section of a membrane with a gold layer anda cross section of an equivalent gold membrane.

FIGS. 9A to 9D illustrate deflection of a membrane.

FIG. 10 shows out of plane deflection of a membrane as a function ofmembrane length and percentage of gold layer coverage.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Rather than using micro-cantilevers, the current disclosure providessystems and techniques for using membrane structures as physical,chemical, and/or biological sensors.

Membranes may be fabricated using materials with lower elasticity modulithan silicon-based materials that are generally used to fabricatemicro-cantilevers. For example, some metal, ceramic, polymer, or othermaterials may be used. The elasticity moduli of these materials may beappreciably less than the moduli of materials usually used to fabricatemicro-cantilevers. For example, parylene has an elasticity modulus ofabout 3.2 GPa, while silicon nitride (a common cantilever material) hasan elasticity module of about 110 GPa, a difference of almost two ordersof magnitude.

The membrane material may also be chosen to be compatible with theexpected operating environment of the sensor. For example,bio-compatible membrane materials (such as Parylene) may be used forbiological sensing, while appropriately compatible materials may bechosen for chemical or physical sensing.

Referring to FIGS. 2A and 2B, a sensor 200 includes a membrane 210.Membrane 210 may be fixed to a substrate 230 to produce a deflection ofmembrane 210 due to a change in surface stress that reflects the changein surface stress.

For example, membrane 210 may be rectangular, as shown in FIGS. 2A and2B. Referring to FIG. 2A, membrane 210 may be fixed to substrate 230 ata first side 232, a second side 234, a third side 236, and a fourth side238. Referring to FIG. 2B, membrane 210 may be fixed to substrate 230only at first side 232 and second side 234.

Membrane 210 may have a region 240 that is modified physically,chemically, or biologically. For example, region 240 may befunctionalized with one or more probe molecules for sensing one or moretarget molecules. Region 240 may include an intermediate layer such as agold layer, a silicon dioxide layer, or other layer to couple probemolecules with the membrane. Using an appropriate intermediate layer,probe molecules may be coupled with the membrane surface via, forexample, an intermediate thiol or cysteamine group. In someimplementations, an intermediate layer may not be necessary.

Membrane 210 may act as a chemical sensor when region 240 is configuredto experience a change in surface stress in response to a chemicalreaction. For example, region 240 may include a thin oxide or polymercoating. Alternately, membrane 210 may comprise a metal such as gold orpalladium to sense materials including, for example, hydrogen ormercury.

A net change in the surface stress on one side of membrane 210 resultsin an out of plane deflection of the membrane. The resulting deflectionor rotation of the flexible membrane can be measured using opticaltechniques, piezoelectric techniques, piezoresistive techniques, orother techniques. Optical techniques include interferometry or opticalbeam deflection.

Referring to FIG. 3, optical beam deflection may be used to detectmembrane deflection. Light is provided by a laser 370, and reflected offa surface 315 of a membrane 310. The reflected light is sensed using adetector 380, such as a charge-coupled device (CCD) detector orphotosensor array detector (e.g., a CMOS array detector).

When membrane 310 is undeflected, the reflected light is received at aregion A of detector 380. When membrane 310 is deflected, the reflectedlight is received at a region B of detector 380. The relative locationof regions A and B provide a measure of the deflection of membrane 310.

Referring to FIGS. 4A and 4B, interferometry may be used to detectmembrane deflection. A reference surface 450 reflects light from a laser470, which is received in a detector 480. A sensor surface 415 of amembrane 410 also reflects the light from laser 470, which is alsoreceived in detector 480. A first interference pattern is produced whenthe membrane is undeflected. As membrane 410 deflects, the interferencepattern shifts. The changes in the interference pattern may be used todetermine the magnitude of deflection of membrane 410.

Referring to FIG. 4A, a Fabry-Perot interferometry apparatus is shown. Areference surface 450 of a glass plate 455 reflects light from laser 470towards detector 480. Sensor surface 415 also reflects light from laser470 towards detector 480, producing an interference pattern. Theinterference pattern may be used to determine the height h by whichmembrane 410 is deflected from its equilibrium position.

Referring to FIG. 4B, a Michelson interferometry apparatus is shown. Areference surface 450 receives light from a laser 470 via a beamsplitter 475, and reflects the light towards detector 480. Surface 415of membrane 410 also receives light via beam splitter 475 and reflectsthe light towards detector 480. An interference pattern is produced,which may be used to determine the height h by which membrane 410 isdeflected from its equilibrium position.

For improved sensitivity, a large deflection/rotation of the sensingelement is desired. Silicon materials may provide less than optimumdeflection due to their high elasticity modulus. Polymers such asparylene, polyimide, etc., may provide a better choice.

Referring to FIG. 5, a membrane 510 (for example, a parylene membrane)is fixed to a substrate 500 at a first end 512 and a second end 514. Awell 520 in substrate 500 allows membrane 510 to deflect. In order todetermine whether a fluid includes one or more target molecules, themembrane will be exposed to the fluid. Subsequently, target moleculesthat bind with the probe molecules on the surface of membrane 510 causemembrane 510 to deflect.

A layer 530 is provided on a surface region of membrane 510. Forexample, layer 530 may be a gold layer that is compatible with thiolchemistry for attaching probe molecules to the surface of membrane 510.Other layer materials may be used; for example, layer 530 may be asilicon dioxide layer.

FIGS. 6A through 6E show process steps that may be used to fabricate asensor comprising a membrane with an intermediate layer to couple one ormore probe molecules to the membrane. A cross-sectional view and a topview (denoting the line of cross section as A-A) are provided in each ofFIGS. 6A through 6E.

Referring to FIG. 6A, a substrate 600 such as a silicon substrate isprovided. Referring to FIG. 6B, a layer of a membrane material 610 suchas parylene is formed on substrate 600. For example, a parylene layermay be formed on substrate 600 using a room temperature chemical vapordeposition (CVD) technique.

Referring to FIG. 6C, layer 610 is patterned to define a membranestructure 615 bounded by substrate regions 605. For example,photolithography may be used to pattern a photoresist layer on layer610, then the pattern may be transferred to layer 610 using an oxygenplasma etch. Referring to FIG. 6D, a layer 620 of a material (e.g.,gold) is formed on membrane structure 615.

Referring to FIG. 6E, substrate material is etched from regions 605 toform an open space beneath membrane structure 615. For example, whensubstrate 600 includes silicon and layer 610 includes parylene, anisotropic etch that is selective of silicon with respect to parylene maybe used to form the open space. For example, a dry plasma etch or a wetetch may be used. Note that membrane structure 615 is not separate fromthe rest of layer 610; rather the membrane structure 615 refers to theportion of layer bridging the open space and fixed to the substrateproximate to the well.

Referring to FIGS. 7A and 7B, a system for performing biological sensingwith multiple sensors is shown. Referring to FIG. 7B, a first sensor710A and a second sensor 710B may be formed in a substrate 700 asdescribed above and shown in FIGS. 6A through 6E. A cover 730, which maybe fabricated from glass or other material, couples with substrate 700to form regions 760A and 760B. First sensor 710A and second sensor 710Bmay deflect in regions 760A and 760B.

Fluid may be provided to and/or removed from regions proximate tosensors 710A and 710B using channels (not shown) that may be formed, forexample, in substrate 700 or in cover 730. Fluid may be provided tosensors 710A and 710B for functionalization; that is, to provide probemolecules for detecting target molecules. First sensor 710A and secondsensor 710B may be functionalized to detect different target molecules,or to detect the same target molecules. Alternately, at least one offirst sensor 710A and second sensor 710B may be used for common moderejection, and may not be functionalized.

For common mode rejection, the deflection of a reference sensor may bemonitored. The deflection of the reference sensor may change over timedue to, for example, a drift in temperature. Since the same drift may beoccurring in other sensors proximate to the reference sensor andintroducing noise into the measurements, the change in deflection of thereference sensor may be used to subtract noise from the other sensors.

Fluid may be provided to sensors 710A and/or 710B to determine whetherthe fluid (i.e., gas or liquid) includes one or more target molecules.The same fluid may be provided to sensors 710A and 710B, or differentfluids may be provided.

Table 1 below includes a list of parameters used in the analysis below.

TABLE 1 Symbol Parameter L₁ Length of polymer membrane w₁ Width ofpolymer membrane t₁ Thickness of polymer membrane I₁ Bending moment ofinertia of polymer membrane (=W₁t₁ ³/12) A₁ Cross-sectional area ofpolymer membrane = w₁t₁ E₁ Elastic modulus of polymer membrane ν₁Poisson's ratio of polymer membrane L₂ Length of gold coating w₂ Widthof gold coating (w_(2 = w) ₁) t₂ Thickness of gold coating I_(eff)Bending moment of inertia of the equivalent gold beam A₂ Cross-sectionalarea of gold coating = w₂t₂ E₂ Elastic modulus of gold coating ν₂Poisson's ratio of gold coating γ Change in surface stress (N/m) y⁺ Ycoordinate of the topmost fiber of the composite membrane y⁻ Ycoordinate of the bottommost fiber of the composite membrane y Ycoordinate of the neutral axis K Curvature of the membrane ε⁺ Strain atthe topmost fiber E^(S) Surface energy per unit length of the membraneE^(B) Elastic strain energy per unit length of the membrane E^(T) Totalenergy of the system per unit length δ_(m) Maximum out-of-planedeflection of the membrane

The following calculations illustrate the benefits that may be obtainedusing a composite membrane such as a parylene membrane with a goldlayer. First, the change in curvature of the gold-covered portion of themembrane as a result of the surface stress change is evaluated. Tofacilitate calculation, an equivalent membrane made of gold was used fora model for the gold-covered portions of the membrane, as shown in FIGS.8A and 8B.

Referring to FIG. 8B, the position of a neutral axis y can be determinedusing Equation (1) below. The y-coordinates of the topmost (y⁺) andbottommost (y⁻) fibers of the composite membrane with respect to theneutral axis are given by Equations (2) and (3). The strain at the topfiber is given by Equation (4), while the surface energy per unit lengthdue to the change in surface stress is given by Equation (5).

$\begin{matrix}{\overset{\_}{y} = \frac{{\frac{E_{1}}{2E_{2}}t_{1}^{2}} + {t_{1}t_{2}} + {\frac{1}{2}t_{2}^{2}}}{{\frac{E_{1}}{E_{2}}t_{1}} + t_{2}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$y⁻=− y  Equation (2)

y ⁺ =t ₁ +t ₂ − y   Equation (3)

ε⁺=Ky⁺  Equation (4)

E^(S)=−γε⁺w₂=−γKy⁺w₂  Equation (5)

The elastic strain energy of the membrane due to bending per unit lengthof the membrane is given by Equation (6). The width of the membrane as afunction of y is given in Equations (7A) and (7B). The curvature K isobtained by minimizing the total energy of the system, given in Equation(8), as shown in Equation (9).

$\begin{matrix}{E^{B} = {\int_{y^{-}}^{y^{+}}{\frac{{E_{2}({Ky})}^{2}}{2\left( {1 - v^{2}} \right)}{w(y)}{y}}}} & {{Equation}\mspace{14mu} (6)} \\{{w(y)} = {\frac{E_{1}}{E_{2}}w_{1}\mspace{14mu} \left( {{- \overset{\_}{y}} < y < {t_{1} - \overset{\_}{y}}} \right)}} & {{Equation}\mspace{14mu} \left( {7A} \right)} \\{{w(y)} = {w_{1}\mspace{14mu} \left( {{t_{1} - \overset{\_}{y}} < y < {t_{1} + t_{2} - \overset{\_}{y}}} \right)}} & {{Equation}\mspace{14mu} \left( {7B} \right)} \\{E^{T} = {E^{S} + E^{B}}} & {{Equation}\mspace{14mu} (8)} \\{K = \frac{{\gamma \left( {1 - v^{2}} \right)}y^{+}}{E_{2}\begin{bmatrix}{\frac{\left( y^{+} \right)^{3}}{3} +} \\{{\left( {\frac{E_{1}}{E_{2}} - 1} \right)\frac{\left( {t_{1} - \overset{\_}{y}} \right)^{3}}{3}} - \frac{{E_{1}\left( y^{-} \right)}^{3}}{3E_{2}}}\end{bmatrix}}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

The vertical deformation of the center of the membrane is evaluatedusing energy minimization and superposition methods. Referring to FIGS.9A-9D, a membrane 910 is fixed to a substrate 900 at either end.Membrane 910 has a gold layer 920 formed symmetrically about an axis 940through the middle of membrane 910.

Referring to FIG. 9B, to determine the net vertical deformation, thevertical deformation due to surface stress change is determined, wherethe deformation of the center of the membrane with respect to the end isgiven by Equation (10) below, and the angle at the free end is given byEquation (11).

$\begin{matrix}{\delta_{1} = {\frac{{KL}_{2}\left( {L_{1} - L_{2}} \right)}{4} + \frac{{KL}_{2}^{2}}{8}}} & {{Equation}\mspace{14mu} (10)} \\{\theta_{1} = \frac{{KL}_{2}}{2}} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

Referring to FIG. 9C, the fixed boundary conditions at the membrane endsrequire that the deformation and the slope at that end are zero.Therefore, there is a moment M (to be determined) at the end to make theslope zero. The deformation of the center of the membrane with moment Mbeing the only load is given by Equation (12), and the correspondingangle at the free end is given by Equation (13).

$\begin{matrix}{\delta_{2} = {\frac{{ML}_{2}^{2}}{8E_{2}I_{eff}} + \frac{{M\left( {L_{1} - L_{2}} \right)}^{2}}{8E_{1}I_{1}} + \frac{{ML}_{2}\left( {L_{1} - L_{2}} \right)}{4E_{2}I_{eff}}}} & {{Equation}\mspace{14mu} (12)} \\{\theta_{2} = {\frac{{ML}_{2}}{2E_{2}I_{eff}} + \frac{M\left( {L_{1} - L_{2}} \right)}{2E_{1}I_{1}}}} & {{Equation}\mspace{14mu} (13)}\end{matrix}$

Referring to FIG. 9D, the deformation of the center of the membrane withrespect to the ends is given by a superposition of the solutions inEquations (10) and (11), as shown in Equation (14) below.

δ_(m)=δ₁−δ₂  Equation (14)

The net angle is constrained to be zero, as shown in Equation (15).Using this relationship, the moment M may be calculated, as shown inEquation (16).

$\begin{matrix}{{\theta_{1} - \theta_{2}} = 0} & {{Equation}\mspace{14mu} (15)} \\{M = \frac{{KL}_{2}}{\frac{L_{2}}{E_{2}I_{eff}} + \frac{\left( {L_{1} - L_{2}} \right)}{E_{1}I_{1}}}} & {{Equation}\mspace{14mu} (16)}\end{matrix}$

Material properties and dimensions for an exemplary composite membraneare given in Table 2 below. The membrane center deformation is afunction of the membrane length and the length of the gold-coveredportion.

TABLE 2 Parylene Gold w₁ 50 μm w₂ 50 μm t₁ 0.5 μm t₂ 0.025 μm E₁ 3.2 GpaE₂ 80 GPa ν₁ 0.3 ν₂ 0.3

FIG. 10 is a plot of the out of plane deflection of a membrane with theparameters given in Table 2 as a function of membrane length and thepercentage of gold coverage. For shorter membranes, the maximumdeflection is obtained with a lower percentage of the membrane coveredby a gold layer than for longer membranes. For a membrane length ofabout 600 μm with 65% gold coverage, the deflection is about 20 nm,assuming a surface stress change of about 1 mJ/m². This deflection maybe measured using the optical detection techniques described above.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, differentmaterials may be used for the membrane. Different layer materials may beprovided on the membrane. Further different methods for providing probemolecules may also be used.

Although rectangular membranes have been shown, other shapes may beused. For example, circular membranes may be used. The shape of themembrane need not be regular or symmetric; a membrane shape thatdeflects in response to a change in surface stress may be used. Theplacement of a layer on the membrane need not be symmetric. Further, themembrane may be attached to the one or more support structures (e.g.,the substrate) differently than shown. Accordingly, other embodimentsare within the scope of the following claims.

1. A sensor, comprising: a membrane to deflect in response to a changein surface stress; and a layer on the membrane, the layer to couple oneor more probe molecules with the membrane.
 2. The sensor of claim 1,further including one or more probe molecules coupled with the membrane.3. The sensor of claim 1, wherein the membrane includes a first portionfixed to a region of a substrate and a second portion fixed to adifferent region of the substrate.
 4. The sensor of claim 1, wherein themembrane comprises a polymer.
 5. The sensor of claim 4, wherein thepolymer is chosen from the group consisting of polyimide and parylene.6. The sensor of claim 1, wherein substantially all of a perimeterregion of the membrane is fixed to one or more structures.
 7. The sensorof claim 1, wherein the layer comprises a material chosen from the groupconsisting of gold and silicon dioxide.
 8. The sensor of claim 1,wherein a portion of a first side of the membrane is covered by thelayer.
 9. The sensor of claim 8, wherein the portion is between about 5%and about 90%.
 10. The sensor of claim 8, wherein the portion is betweenabout 10% and about 70%.
 11. A system, comprising: a substrate; amembrane coupled with the substrate and spanning a well in thesubstrate; a layer on the membrane, the layer to couple one or moreprobe molecules with the membrane; another membrane coupled with thesubstrate and spanning another well in the substrate; a layer on theanother membrane, the layer to couple one or more probe molecules withthe membrane; a cover to enclose the well and the another well; achannel to provide fluid to the membrane; and another channel to providefluid to the another membrane. 12.-19. (canceled)
 20. A method,comprising: introducing fluid into a region proximate to a membrane, thefluid including one or more target molecules to be sensed, at least someof the one or more target molecules subsequently interacting with one ormore probe molecules coupled with a layer on the membrane and causing adeflection of the membrane; and measuring the deflection of themembrane.
 21. The method of claim 20, further including introducing adifferent fluid into another region proximate a different membrane, theanother fluid including one or more different target molecules to besensed, at least some of the one or more different target moleculessubsequently interacting with one or more different probe moleculescoupled with a layer on the different membrane and causing a deflectionof the different membrane; and measuring the deflection of the differentmembrane.
 22. The method of claim 20, wherein measuring a deflection ofthe membrane comprises using an electrical detection method.
 23. Themethod of claim 22, wherein the electrical detection method is chosenfrom the group consisting of a piezoelectric detection method and apiezoresistive detection method.
 24. The method of claim 20, whereinmeasuring a deflection of the membrane comprises reflecting light offthe membrane.
 25. The method of claim 20, wherein measuring a deflectionof the membrane further comprises reflecting light off a referencesurface.
 26. The method of claim 20, wherein measuring a deflection ofthe membrane comprises using Fabry-Perot interferometry.
 27. The methodof claim 20, wherein measuring a deflection of the membrane comprisesusing Michelson interferometry.